The present invention relates to a servo-assisted control system of the electro-hydraulic type operable to control the shift between the various gears of a double clutch gearbox of a motor vehicle, in particular a six- or seven-speed gearbox.
A gear control system is known from U.S. patent application No. 013417 in the name of the Applicant, which is able to control the displacement of the four engagement sleeves of a six-speed gearbox of a motor vehicle, whether it is of the double clutch type or of the single clutch type with robotized control. This known system fundamentally comprises:                a first control device for controlling the displacement of the coupling sleeves associated with the gears controlled by the first input shaft of the gearbox, that is to say the first, third, fifth and sixth gear, as well as the reverse gear; and        a second control device for controlling the displacement of the coupling sleeve associated with the gears controlled by the second input shaft of the gearbox, that is to say the second and fourth gear.        
The first control device is provided with a drum which is mounted rotatably about its own axis and on the cylindrical lateral surface of which are provided three control grooves each of which engages a respective pin to displace it in the direction of the axis of the drum upon rotation of this latter. The three pins are each connected to a respective fork which controls the displacement of a respective coupling sleeve. The second control device is provided with a slidable rod carrying a fork for control of the displacement of the coupling sleeve of the second and fourth gear.
According to a first embodiment this known control system is electrohydraulically operated. The two control devices are alternatively controlled by a first proportional solenoid valve which controls the up shifting, and by a second proportional solenoid valve which controls the down shifting. The two solenoid valves modulate the pressure of the working fluid supplied by a pump in a delivery line and alternatively connect a first and a second input line of a six-way distributor with the delivery line from the pump or with a discharge line. The six-way distributor is further connected to the hydraulic actuator of the first control device through third and fourth output lines and to the hydraulic actuator of the second control device through fifth and sixth output lines.
This known control system, in combination with the double clutch six-speed gearbox described in the above document, makes it possible to perform multiple gear changes in “power shift” mode during the following downshift maneuvers: from sixth to fourth or to second; from fifth to second; and from fourth to first. The remaining multiple downshift maneuvers can however be performed in a traditional manner, that is with interruption of torque transmission.
A further example of a servo-assisted control system for the gears of a six-speed double clutch gearbox for a motor vehicle is known from German Patent Application DE 101 34 115. This known control system includes a hydraulic circuit arranged to control four double-acting hydraulic cylinders for actuation of four coupling sleeves, that is to say a first sleeve which effects engagement of the first or third gear, a second sleeve which effects engagement of the fifth gear, a third sleeve which effects engagement of the second or fourth gear and a fourth sleeve which effects engagement of the sixth gear or the reverse gear. The hydraulic circuit is subdivided into a first portion intended to control the odd gears and a second portion intended to control the even gears and the reverse gear. Upstream of each circuit portion is disposed a pilot valve which controls the supply of oil under pressure to the respective circuit portion. Each circuit portion includes a pair of proportional solenoid valves which control the two double-acting hydraulic cylinders to actuate the coupling sleeves of the gears associated with this circuit portion. Between the four double-acting hydraulic cylinders and the four proportional solenoid valves associated therewith is interposed a distributor.
This known control system makes it possible to perform multiple gear changes in “power shift” mode directly (that is non-sequentially) between gears not associated with the same input shaft of the gearbox. It has, however, the disadvantage of requiring a large number of components and of therefore having a high cost.